The present invention provides a method for producing sintered bodies or coatings from metal or ceramic powder, using a suspension of surface-modified nanoscale metal or ceramic particles.
"Nanoscale particles" are hereinafter to be understood to mean particles (including powders), whose average size is not more than 10 nm, in particular not more than 50 nm and particularly preferably not more than 30 nm. "Nanodisperse materials" are nanoscale particles dispersed in a cartier medium which can be a binder and may include dispersion aids.
In the processing of nanodisperse materials there are essentially two problems, namely
(a) the regulation of particle agglomeration in the processing of these materials and PA1 (b) the production of processable ceramic materials with high solids contents. PA1 wear protection of metals in abrasive and tribological applications, PA1 on cutting, drilling and milling tools for increasing the machining capacity, PA1 as corrosion protection coatings in chemical reactors, PA1 as a coating of watch cases and jewellery. PA1 bulk ceramic, for example for abrasive powder; PA1 coating material for metals, ceramics and glass for decoration purposes, wear protection, triboligical applications, corrosion protection, in particular as a coating on cutting tools and abrasive agents or abrasive powders; PA1 component in ceramic/ceramic composites. Al.sub.2 O.sub.3, TiC, SiC and Si.sub.3 N.sub.4, in particular, come into consideration as matrix phase; PA1 component of nanocomposites; PA1 sintering aids for relatively coarse ceramics; PA1 hard-type metal/ceramics composites; PA1 cermets; PA1 microporous coatings for filtration purposes, e.g. micro-ultra-nano-filtration and reverse osmosis.
Regarding problem (a), it is evident that, in the transition from submicron to nanoscale powders, an increase in agglomeration is generally observed. This can be attributed to the fact that, with decreasing particle size, weak forces of interaction, for example van der Waals forces, also gain considerably in importance, or even com to predominate. In addition, there is the fact that the particle surface is always occupied by functional groups, that is to say groups capable of undergoing condensation. In conventional submicron powders, these groups are only of significance to the extent that they can be used as centers of interaction for necessary organic processing aids (dispersion aids, binders etc.). Because of the large surface-area-to-volume ratio of nanodisperse materials, however, the surface groups also take on great importance from another point of view. On the one hand they similarly serve as reaction centers for organic processing aids. On the other hand, however, they can also lead to the formation of hard agglomerates as a result of condensation reactions taking place between individual particles. The particles are then joined to one another by, so to speak, sinter bridges. It is therefore desirable to develop methods with which the agglomeration can be controlled in such a manner that powders agglomerated in a regulated manner can be obtained. Furthermore, it would be desirable if, with this method, the reactive surface could be outwardly shielded, and interparticle condensation thus be prevented.
Regarding the aforementioned problem (b), it is notable that the production of ceramic compounds with high solids contents and processing properties matched to a shaping process poses serious difficulties. To avoid agglomerated material, which may lead to severe defects in both green and sintered bodies, the materials are generally used in suspensions. For suspension stabilization, dispersion aids are generally added, which have the function of preventing agglomeration and providing the suspension with the necessary processing properties. For suspension stabilization, two principal procedures can generally be distinguished, namely electrostatic and stabilization and steric stabilization.
Electrostatic stabilization has the disadvantage that, by virtue of the relative large hydrodynamic radius of the suspended nanoscale particles, only small solids contents are feasible. Steric stabilization, by contrast, provides the possibility, in principle, of producing suspensions with high solids contents from nanoscale materials, since in this case the hydrodynamic particle radius is much smaller.
The advantage of steric stabilization have already been indicated with reference to the example of SiO.sub.2. In this case, for the dispersion aid, nonionic organic polymers (e.g. polymethylmethacrylate) were generally used, which are adsorbed on the particle surface. The disadvantage of this kind of stabilization is that in this case, too, maximum solids contents of approx. 20 to 30 vol.-% are generally only feasible, and it is only possible to apply it to materials systems different from SiO.sub.2 with considerable restrictions. This is in particular because the surfacechemical properties (e.g. acid/basic properties) specific to a material usually cannot be taken into account.
It is therefore desirable to provide a method with which it is possible to modify the particle surface by means of suitable chemical compounds such that an optimum degree of dispersion is achieved and high solids contents of the dispersion are feasible.
For example, titanium nitride (TIN) falls within the group of metallic hard materials and has a cubic crystalline structure. Because of the high proportion of covalent bonding, TiN has a high melting point, a high hardness and good oxidation resistance and corrosion resistance. These properties are the reason the for applications of TiN as coating materials for wear protection on metals and as one of the components in multiphase ceramics, for example Al.sub.2 O.sub.3 /TiN or Si.sub.3 N.sub.4 /TiN.
Pure TiN coatings or TiN coatings with admixtures of TiC are today produced via gas-phase processes. These include the CVD (chemical vapour deposition) and PVD (physical vapour deposition) processes. Corresponding apparatus is commercially available and a component of industrial production processes. These coatings are used in the following fields:
A disadavantage of the TiN coatings produced by, for example, CVD and PVD is the inadequate adhesion to the substrates, so that the coatings often flake off and tools coated therewith become prematurely unusable. Substrates that can be used are metals with high heat resistance, hard metals, for example WC/Co, or else ceramic inserts.
Another application of nanocrystalline (nanoscale crystalline), ceramic powders such as TiN, TiC, SiC is their use in composite ceramics, for example Al.sub.2 O.sub.3 /TiC or Si.sub.3 N.sub.4 /TiN. The addition of such powders to the matrix materials can improve their mechanical properties, for example hardness, toughness or compressive strength. In a similar manner, the mechanical properties of bulk ceramics and metallic materials produced by powder metallurgical methods can be considerably improved by the application of nanocrystalline powders.
For example, by virtue of its high covalent bonding characteristic, pure TiN has only a very low sinter activity. Compaction therefore normally requires the use of sintering additives. In the simpliest case this may be TiO.sub.2 which is formed on the TiN surface in air in the presence of water. For example, it has been reported that TiN powder with an average grain size of 0.1 .mu.m can be sintered without pressure at temperatures of about 1500.degree. C. up to relative densities of 95%. This sintering behaviour is ascribed to the activation of the diffusion mechanisms leading to compaction by the break up ot TiO.sub.2 localized on the TiN particle surface. Various publications deal with the sintering of TiN under pressure and/or in the presence of sintering additives. Thus, the hot pressing of TiN powders with a ds.sub.50 value of 1 .mu.m at temperatures up to 2100.degree. C. and a sintering pressure of 14 MPa only leads to a density of 93% of the theoretical density of TiN; see M. Morijama et at., "Mechanical and Electrical Properties of Hot-Pressed TiN-Ceramics without Additives", J. Jap. Ceram. Soc., 22 (1991), pages 275-281. In M. Morijama et al., "The Mechanical Properties of Hot-Pressed TiN Ceramics with Various Additives", J, Jap. Ceram. Soc., 101 (1993), pages 271-276, the compaction behaviour of TiN in the presence of sintering additives during hot pressing is described. Specimens with a total of 10 wt.-% of Al.sub.2 O.sub.3, Y.sub.2 O.sub.3 and B.sub.4 C produce, after hot pressing at 1950.degree. C. and 14 MPa, densities of around 97 % of the theoretical. Furthermore, a 95% compaction by hot pressing at 1800.degree. C. and 5.0 GPa has been reported.
The object of the present invention is to provide a method for producing metal and ceramic sintered bodies and coatings, which makes possible a regulation of the particle agglomeration and sufficiently high solids contents of the particle suspension used and can be carried out at relatively low sintering temperatures.